Industry observers say that the low cost of natural gas, as well as regulatory actions by the Environmental Protection Agency, could temper the immediate impact of the Supreme Court's April 29 ruling that upheld an air-quality rule for emissions that move across state lines.

Still, the EPA contends the Cross-State Air Pollution Rule (CSAPR) would require some $800 million annually in powerplant retrofits, mostly in upwind states. Some sources argue the court's decision may embolden the agency to interpret its Clean Air Act authority broadly in future regulatory actions.